1. Field of Endeavor
The invention relates to a device for cleaning oxidized or corroded components in the presence of a halogenous gas mixture, with a cleaning retort into which, indirectly or directly, leads a feed line which is connected via a flow control unit to a gas reservoir which stores the halogenous gas mixture. In particular, these components can be turbine components, especially gas turbine blades, which are exposed to impingement by hot gases.
2. Brief Description of the Related Art
Turbine components for power plants or stationary gas turbine installations, which are indirectly or directly exposed to hot gas flows, such as stator blades or rotor blades, heat accumulating segments or similar components or component groups which delimit the hot gas passage, are subjected to operation-induced material degradations which frequently lead to cracks and to mechanical weakening of the respective components which is associated with them. On account of the high temperature, stresses and pressure stresses which prevail in the hot gas passages, to which the corresponding components, which are mostly manufactured from nickel-based materials, are exposed, complex chemically and thermally stable oxides, as result of external and internal oxidation, are deposited on the surfaces of the components, as operating time increases, within the developing crack openings, and also within the regions inside the base material which lie close to the surface.
With a special process chain, it is the aim to change the components, which are stressed in this way and partly damaged into a condition, which largely corresponds to the condition of a newly manufactured comparable component. In this case, it is one of the steps to carefully clean the component which is to be reworked, i.e., to remove the complex oxide layer which is deposited on the surface of the component, and also to remove cracks which have developed within it, without damaging the material of the component itself in the process.
A corresponding cleaning process for the previously referenced components is described in DE 28 10 598 A1, which components are exposed to a pressurized cleaning atmosphere at temperatures of above 1000° C., in which gaseous active fluoride ions are contained. In the presence of such a cleaning atmosphere, the complex oxide reacts with the fluoride ions, forming a gaseous fluoride, without damaging the component material in the process. Such cleaning processes, which are also generally referred to as FIC (fluoride ion cleaning), are sufficiently well-known and described in many publications. In this connection, EP 0 209 307 B1 may be representatively referred to, from which a respectable overview of the cleaning technologies which are known up to now can be gathered.
The object of completely removing oxide layer portions which have been deposited, especially in crack-induced fissure or fault structures, is largely common to all efforts for improving such FIC processes, particularly as the smallest residual portions of oxidized or corroded surfaces already have long-term effects on subsequent repair measures. For purposes of crack-repair on the respectively cleaned components, a soldering or welding process is typically carried out in such a way that a repair alloy in powder form is accumulated over a cleaned crack and, in the presence of a vacuum and by heat action, is made to melt and finally to flow into the fissure-like crack. In this case, a wetting of the crack wall with liquefied repair alloy is formed. It is plain that corresponding wettings on a component surface which is not afflicted with an oxide layer are not carried out, or carried out to a far lesser extent, as a result of which repair weak spots ultimately result, which is necessary to avoid.
For improving the cleaning result, in the previously cited EP 0 209 307 B1, it is proposed to cyclically vary the pressure within the reactive cleaning atmosphere in order to create in this way a common movement of the reactive gaseous fluoride ions in the region of a component which is to be cleaned for the purpose of an intimately contacting the gaseous reaction agent with the walls of the cracks and cavities within the damaged component.
In DE 10 2005 051 310 A1, it is proposed to periodically purge the reaction chamber, in which a halogenous gas mixture is introduced for purposes of component cleaning, with a non-halogenous gas during the cleaning.
U.S. Pat. No. 6,536,135 B2 describes a FIC process in which an improved oxide cleaning is undertaken by variation of the partial pressures of the cleaning gas mixture by carbon being added as an additional component to the cleaning gas mixture, which consists of hydrogen fluoride (HF) and hydrogen (H2). The carbon is added in the form of different compounds which form a carbonaceous gas during the process.
Furthermore, a typical cleaning retort, which provides a cylindrical housing which can be closed off from the top in a gastight manner and which, in an opened state, can be loaded from the top with components which are to be cleaned, is to be gathered from the publication. The components which are to be cleaned are accommodated on racks, so-called trays, which are provided vertically one above the other and fastened on a central pipe which is arranged centrally in the cleaning retort and through which a carbon-enriched hydrogen fluoride-gas mixture is fed to the cleaning retort. The central pipe which penetrates the retort head in a gastight manner extends vertically inside the cleaning retort downwards into the region of the so-called retort sump, in which the central pipe provides a gas distribution structure which extends essentially over the entire cross section of the cleaning retort, and with discharge openings via which the halogenous cleaning gas mixture is fed into the cleaning retort in a manner in which it rises from the bottom upwards. In doing so, the cleaning gas mixture flows through the entire retort volume from the retort sump in the direction of the retort head on which a corresponding gas outlet opening is provided.
ALSTOM has longstanding practical experience in the field of cleaning gas turbine components of the previously described type, which for operationally induced reasons are contaminated, corroded, oxidized, and degraded, especially using FIC cleaning processes and also the cleaning plants which are required for it. In the longstanding association with a cleaning retort which relates to this, which via a central pipe is fed with a cleaning gas mixture which contains hydrogen fluoride and hydrogen in varying ratios, it has been shown that significant malfunctions in the cleaning process are created as a result of volume fluctuations in the feed of the cleaning gas into the cleaning retort, which, upon exceeding certain proportions, can occasionally lead to the breakdown of the entire cleaning process. More accurate investigations furthermore showed that the feeding of fluctuating hydrogen fluoride gas volumes inside the cleaning retort lead to concentration fluctuations which ultimately result in a reduced cleaning efficiency and in an inaccurately controllable cleaning quality which is associated with it. In particular, in the case of very badly damaged components with a large number of material cracks which furthermore have a wide spectrum with regard to depth, width, and length of the individual cracks, an intended level of cleaning under these circumstances can no longer be ensured. The consequences of an incomplete cleaning of components which are covered with a layer of complex oxides have already been discussed above.
A further disadvantageous and therefore improvement-deficient aspect in the case of the cleaning practices which have been used up to now relates to the construction of the cleaning retort. On the one hand, the inflow of cleaning gas into the cleaning retort by means of the centrally guided central pipe and by means of a distribution structure which is provided in the bottom sump region of the retort, and the positioning and stacking possibilities of the individual components which are to be cleaned on the stacking trays which are provided along the central pipe in a vertical sequence, have already been described in connection with the aforementioned U.S. Pat. No. 6,536,135 B2. On account of such a known construction, the stacking or positioning possibilities for the individual components which are to be cleaned in the cleaning retort are limited. The equally improvement-deficient ratios of flow onto the individual components which are to be cleaned inside the cleaning retort add to this, particularly as it cannot be ruled out that, on account of a mutual masking of specific surface regions on the components which are to be cleaned, only an inadequate impingement with cleaning gas is carried out. Thus, the fact that regions with comparatively poor flow ratios and concentration ratios, up to the point of there being dead water regions, are formed as a result of a cleaning gas feed which is provided exclusively in the retort sump region, as a result of which a smaller gas exchange is initiated, especially in regions of the cracks, is not to be ruled out.
For countering the previously pointed out problems with regard to the improvement of cleaning quality, attempts have been made to increase the cleaning cycle times in order to maintain a longer interaction period between the components which are to be cleaned and the cleaning gas mixture, produced only inconsequential results. Moreover, cleaning processes with an increased HF concentration were carried out. However, these attempts only showed that the set cleaning aims were not focused to a satisfactory degree. Rather, these measures led to a cost increase and also to an increased material attack on the components which are to be cleaned.